In recent years, surface acoustic wave devices have found wide application in communications devices such as motor vehicle telephones as circuit elements of resonator filters, signal processing delay lines, etc. For example, FIG. 5 shows a surface acoustic wave device comprising interdigital electrodes 2, 3 and lattice like reflectors (not shown) which are formed on the surface of a piezoelectric substrate 1. The device converts electric signals to surface acoustic waves and vice versa.
Surface acoustic waves are surface waves which literally propagate along the surface of an elastic body, and the energy thereof is not radiated into the substrate. A plurality of modes of excitation have been discovered as such surface acoustic waves. For example, already known are the Rayleigh wave, Sezawa wave, Love wave, electroacoustic wave, etc.
In the Rayleigh and Sezawa waves, predominant are both a longitudinal wave component having a displacement in the same direction as the direction of propagation and a transverse wave component having a displacement depthwise of the substrate. On the other hand, predominant in the Love wave and electroacoustic wave is a transverse component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction. While three kinds of bulk waves, i.e., "slow transverse wave," "rapid transverse wave" and "longitudinal wave" are usually present in the piezoelectric substrate, the surface acoustic waves propagate at a phase velocity lower than that of the "slow transverse wave."
Also known are elastic waves which propagate along the surface of an elastic body while radiating energy depthwise of the body. These waves are called quasi surface acoustic waves or leaky surface acoustic waves. The quasi surface acoustic wave initially discovered comprises a predominant transverse wave component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction, and is intermediate between the "slow transverse wave" and the "rapid transverse wave" in phases velocity.
Quasi surface acoustic waves having a predominant longitudinal wave component are discovered in recent years one after another (see JP-A-112763/1994 and Proceedings of the 15th Symposium on Fundamentals and Applications of Ultrasonic Electronics, 1994, pp. 185-186). These quasi surface acoustic waves having a predominant longitudinal wave component are intermediate between the "rapid transverse wave" and the "longitudinal wave."
On the other hand, there is a case wherein bulk waves propagating along and close to the surface of a substrate are excited by interdigital electrodes and detected by other interdigital electrodes on the same substrate. Such bulk waves are termed surface skimming bulk waves. It is thought that there are three kinds of surface skimming bulk waves in corresponding relation with the usual bulk waves. However, mainly handled at present is the surface skimming bulk wave which comprises a predominant transverse wave component having a displacement in parallel to the substrate surface and perpendicular to the propagation direction.
The characteristics of elastic waves include acoustic velocity, propagation loss and electromechanical coupling factor. These characteristics relate directly to the design parameters of the circuit to which the surface acoustic wave device is applied.
The period T of the electrode digits of interdigital electrodes or lattice like reflectors has a value equivalent to the wavelength of elastic waves, so that at a constant frequency, the lower the acoustic velocity, the smaller is the wavelength and the more difficult are the electrodes to fabricate. It is therefore desired that the acoustic velocity be higher.
The resonance sharpness of surface acoustic wave resonators and the insertion loss of surface acoustic wave filters are dependent directly on the propagation loss of surface acoustic waves. For this reason, the propagation loss should preferably be small.
On the other hand, the electromechanical coupling factor represents the ratio of conversion of the energy of an input electric signal into the energy of surface acoustic waves. When the interdigital electrodes have a sufficiently increased number of electrode digits, elastic waves of desired energy can be excited even if the electromechanical coupling factor is small, whereas the interdigital electrodes then have an increased electrical capacity, which presents difficulty in impedance matching with an external circuit, necessitating an additional matching circuit for impedance matching. Further it is known that the number of electrode digits of interdigital electrodes is approximately in inverse proportion to the operating frequency range of the surface acoustic wave device, such that an increase in the number of electrode digits limits the realizable characteristics to a narrow frequency range. Accordingly, the electromechanical coupling factor is preferably great.
Already known are substrate conditions (e.g., the relationship between the crystal axis and the direction of propagation of surface acoustic waves) and electrode conditions (e.g., the period of electrode digits and film thickness of the electrode digits) for improving the foregoing characteristics in connection with elastic waves (such as the Rayleigh wave and Sezawa wave) which comprise two predominant components, i.e., a transverse wave component having a depthwise displacement and a longitudinal wave component, and elastic waves (such as the electroacoustic wave, Love wave, quasi surface acoustic wave of the transverse wave type and surface skimming bulk wave of the transverse wave type) which comprise a predominant transverse wave component having a displacement in parallel to the surface and perpendicular to the propagation direction Proceedings of the 1994 IEICE (Institute of Electronics, Information and Communication Engineers) Spring Conference, "A-437," "A-438," Japanese Journal of Applied Physics, vol. 29(1990) Supplement 29-1, pp. 119-121, Japanese Journal of Applied Physics, vol. 30(1991) Supplement 30-1, pp. 143-145, etc.!.
However, the requirements of the electrodes for improving the above characteristics still remain to be clarified for surface acoustic waves wherein the longitudinal wave component predominates over the transverse wave component (longitudinal wave-type surface acoustic waves), quasi surface acoustic waves wherein the longitudinal wave component predominates over the transverse wave component (longitudinal wave-type quasi surface acoustic waves) and surface skimming bulk waves wherein the longitudinal wave component predominates over the transverse wave component (longitudinal wave-type surface skimming bulk waves).
Especially, longitudinal wave-type quasi surface acoustic waves have an acoustic velocity in excess of 6000 m/s and an electromechanical coupling factor of greater than 2%, and are therefore advantageous to use in surface acoustic wave devices in this respect, whereas conventional surface acoustic wave devices for use with longitudinal wave-type surface acoustic waves are as great as 0.5 dB per wavelength in propagation loss. Such an extremely great loss has been an obstacle to the actual use of the waves of the type mentioned.